This invention relates to the preparation of phosphorus oxyacids, and more particularly to novel processes for the preparation of oxyacids by catalytic reaction of water and elemental phosphorus.
Oxyacids of phosphorus are important precursors for the synthesis of other phosphorus species having various applications, for example, in herbicides, insecticides, fertilizers, flame retardants and plasticizers.
Phosphoric acid for use in fertilizer manufacture is conventionally prepared by acidulation of phosphate rock with sulfuric acid, resulting in substantial generation of by-product gypsum or calcium sulfate hemihydrate which must be disposed of either as a by-product or waste material. Environmental and corrosion issues may also arise from the generation of HF by acidulation of fluoride contained within phosphate rock.
Higher purity phosphoric acid is produced by oxidation of elemental phosphorus to phosphorus pentoxide, and absorption of phosphorus pentoxide in dilute phosphoric acid. This process requires a combustion furnace in which phosphorus is burned to phosphorus pentoxide at temperatures in excess of 3500.degree. F., and is generally adapted for the production of phosphoric acid only on a large scale.
Phosphorous acid has been conventionally manufactured by hydrolyzing a halogen derivative of phosphorus, such as phosphorus trichloride. Since the halogen derivatives are prepared from elemental phosphorus, an economic advantage could be realized by preparing phosphorous acid directly from elemental phosphorus. Direct preparation could also provide environmental benefits by avoiding the use of halogen-containing phosphorus starting materials and production of halogen-containing by-products.
As described by Engel, "Oxidation of Hypophosphorous Acid by Hydrogenated Palladium in the Absence of Oxygen," Comet. Rend. Acad. Sci., 1890, pp. 786-787, phosphorous acid can also be prepared by oxidation of hypophosphorous acid with water in the presence of a palladium catalyst. However, commercial processes are not readily available for the economical preparation of the hypophosphorous acid starting material without formation of phosphine or other undesirable by-products.
Christomanos (Z. Anorg. Chem., 41, 305-14, 1904) describes an analytical procedure for determination of elemental phosphorus in organic solutions by a metal induced disproportionation to phosphorous acid and a Cu phosphide: EQU P.sub.4 +CuSO.sub.4 +6 H.sub.2 O.fwdarw.Cu.sub.3 P.sub.2 +3H.sub.2 SO.sub.4 +2H.sub.3 PO.sub.3
Comparable reactions of elemental phosphorus with Cu.sub.2 NO.sub.3 are also disclosed. Only stoichiometric reactions are described. Atmospheric oxygen is said to have an oxidizing function. After four hours, Cu phosphide disappears and the solution contains only Cu phosphate.
White phosphorus, the elemental phosphorus allotrope also referred to as yellow phosphorus or tetraphosphorus (P.sub.4), is a potential starting point for the synthesis of a variety of phosphorus species. The tetrahedral structure of white phosphorus contains six phosphorus-phosphorus bonds and can provide a large number of reactive species having an intermediate existence in phosphorus reactions. As noted, tetraphosphorus is the raw material for one of the major commercial processes for the manufacture of phosphoric acid. If tetraphosphorus could be used as a starting material for the manufacture of other oxyacids of phosphorus without intermediate halogenation, significant economic advantages might be realized, especially if the reaction could be conducted under relatively mild conditions. However, in the exothermic reaction of phosphorus with oxygen, it is difficult to control the reaction short of the formation of the P(V) oxide, i.e., the anhydride of phosphoric acid.
Ipatiev U.S. Pat. Nos. 1,848,295 and 1,895,329 describe processes for the preparation of phosphoric acid by catalytic oxidation of liquid phosphorus with water at high temperature and pressure. Catalysts include salts of copper and nickel, copper or nickel phosphide being formed in the reaction. Ipatiev reports that phosphorous acid is formed as an undesired by-product of the oxidation reaction, particularly early in the reaction, but does not disclose the fraction of phosphorous acid present in the reaction mixture, or the relative proportions of phosphorous and phosphoric acid present, at any time during the reaction. Ipatiev teaches that the reaction is preferably conducted at temperatures of 300.degree. C. or above, but the '329 patent includes an example at 200.degree. C. in which by-product copper phosphide is found in the phosphorus phase at the end of the reaction.
Numerous references describe the preparation of phosphoric acid by catalytic vapor phase oxidation of phosphorus with water at temperatures above 600.degree. C., commonly above 1000.degree. C. Various catalysts are disclosed to for use in these reactions, including copper, silver and a wide variety of other metals, particularly other Group IB and Group VIII metals, certain Group VI metals (e.g., Cr, Mo, W and U), certain Group VII metals (e.g., Mn), and/or their oxides, salts and/or phosphides. To prevent leaching of catalyst out of the reaction zone, it has been proposed to use various supports for active catalysts, including, for example, pyrophosphates of Ti or Zr. Liljenroth U.S. Pat. No. 1,605,960, e.g., also lists noble metals such as Ru, Rh, Pd, Os, Ir or Pt as catalysts for the reaction.